1. Field of the Invention
The present invention relates to a fuel cell stack formed by stacking a plurality of fuel cells each including an electrolyte electrode assembly and a pair of separators for sandwiching the electrolyte electrode assembly. The electrolyte electrode assembly includes a pair of electrodes, and an electrolyte interposed between the electrodes.
2. Description of the Related Art
Generally, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which comprises two electrodes (anode and cathode) and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly is interposed between separators. The membrane electrode assembly and the separators make up a unit of the fuel cell for generating electricity. A predetermined number of fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a hydrogen-containing gas is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. An oxygen-containing gas or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
Some of the fuel cells in the fuel cell stack are cooled down easily due to heat radiation to the outside in comparison with the other fuel cells. For example, fuel cells (end cells) provided at opposite ends of the fuel cell stack radiate heat to the outside through terminal plates for collecting electric energy generated in the fuel cells, and end plates for tightening the fuel cells. The temperature of the end cells is likely to be dropped excessively.
Due to the temperature drop, water vapor may condense into liquid water easily at the end cells in comparison with fuel cells provided in the middle of the fuel cell stack in the stacking direction. The water produced in the reaction in the fuel cell stack may not be discharged smoothly from the fuel cell stack. Thus, the desired power generation performance may not be achieved. In particular, if a coolant flow field for supplying a coolant to the fuel cell stack is provided adjacent to the terminal plate, and operation of the fuel cell stack is started at a temperature below the freezing point, heat energy generated in the end cell is absorbed by the coolant, and transmitted to the terminal plate. Thus, the end cell is not warmed up efficiently. Consequently, voltage drop may occur undesirably.
In an attempt to address the problem, Japanese Laid-Open patent publication No. 8-130028 discloses a solid polymer electrolyte fuel cell in which separators of end cells provided at opposite ends of the fuel cell stack do not have any grooves as passages for cooling fluid. According to the disclosure, since the outer separators are not cooled by the cooling fluid, it is possible to prevent the end cells from being cooled down excessively.
Japanese Laid-Open patent publication 7-326379 discloses another type of a fuel cell stack in which gas connector plates are provided at opposite ends of a cell stack body. A vacuum layer and an air layer are formed in each of the gas connector plates. The vacuum layer and the air layer jointly function as a heat insulation layer for preventing heat radiation from the cell stack body to the outside.
As described above, Japanese Laid-Open patent publication No. 8-130028 is directed to prevent the end cells from being cooled excessively by the cooling fluid so that water condensation does not occur in the end cells, and Japanese Laid-Open patent publication 7-326379 is directed to prevent heat radiation from the cell stack body to the outside by means of the heat insulating function of the vacuum layer and the air layer.
Basically, both of Japanese Laid-Open patent publication No. 8-130028 and Japanese Laid-Open patent publication 7-326379 provide heat insulating mechanisms operated in an atmosphere having a normal temperature, for insulating the end cells and the cell stack body to maintain the desired power generation performance of the end cells and the cell stack body. The heat insulating mechanisms of these techniques do not function properly when operation of the fuel cell stack is started at an excessively low temperature below the freezing point. In starting operation of the fuel cell stack at such a low temperature, it is necessary to rapidly raise the cell temperature to the desired temperature for power generation to prevent water produced in the fuel cell stack from freezing undesirably.
When operation of the fuel cell stack is started at a temperature below the freezing point, the reactant gas flow field in the gas diffusion layer of the membrane electrode assembly may be closed undesirably by the frozen water. It is necessary to rapidly raise the temperature of the gas diffusion layer adjacent the gas flow field above the freezing point. Japanese Laid-Open patent publication No. 8-130028 and Japanese Laid-Open patent publication 7-326379 do not suggest any technique for keeping the temperature of the gas diffusion layer above the freezing point.